Glycine, along with γ-aminobutyric acid (GABA), is primarily responsible for inhibiting neurotransmission in the CNS. Additionally, glycine is an essential co-agonist at the N-methyl-D-aspartate (NMDA) receptor, where it acts to attenuate the excitatory actions of glutamate (L. L. Iverson, Br. J. Pharmacol. 1971, 41(4):571–591).
Radio-labeled strychnine binding studies (A. B. Young and S. H. Snyder, Proc. Natl. Acad. Sci. U.S.A. 1973, 70(10):2832–2836; M. A. Zarbin et al., J. Neurosci. 1981, 1(5):532–547; A. Probst et al., Neuroscience 1986, 17(1):11–35; H. Betz, Nature 1987, 328(16):215–220) provide strong evidence that glycine is the major inhibitory amino acid operating in the brainstem and spinal cord of vertebrates, and exerts its effects post-synaptically at the strychnine-sensitive glycinergic receptor (K. Krnjevic, Physiol. Rev. 1974, 54(2):418–540).
The binding of glycine to its specific receptor induces the opening of a ligand-gated chloride channel, which results in an influx of chloride ion into the post-synaptic neuron. This process causes the neuron to become hyperpolarized, and ultimately raises the threshold for neuronal signaling. The physiological effects of glycine are regulated by glycine transporters, which provide a mechanism for the re-uptake of glycine from the synaptic cleft back into the pre-synaptic neuron and surrounding glial cells.
Currently there are two known glycine transporters expressed in the CNS: GlyT1 and GlyT2 (J. Guastella et al., Proc. Natl. Acad. Sci. USA 1992, 89(15):7189–7193; Q.-R. Liu et al., J. Biol. Chem. 1993, 268(30):22802–22808; B. Lopez-Corcuera et al., J. Neurochem. 1998, 71(5):2211–2219). Separate genes encode each transporter, and the transporters have distinctly different pharmacologies as evidenced by their sensitivities to sarcosine (N-methylglycine) (B. López-Corcuera et al., J. Neurochem. 1998, 71(5):2211–2219). Both the rat and human GlyT2 transporters have been cloned and share ˜93% sequence homology at the amino acid level (M. J. Gallagher et al., Mol. Brain Res. 1999, 70(1):101–115; J. Evans et al., FEBS Lett. 1999, 463(3):301–306). Biochemical evidence gathered thus far suggests that the GlyT2 transporter is closely associated with the strychnine-sensitive glycine receptors in the brainstem and spinal cord.
GlyT2 inhibitors should prevent glycine reuptake and accentuate the post-synaptic inhibitory activity of the glycineric receptor, and may thus be useful in the treatment of CNS conditions associated with glycinergic receptor malfunction, such as muscle spasticity, tinnitus, epilepsy and neuropathic pain (E. Friauf et al., J. Comp. Neurol. 1999, 412(1):17–37; R. K. Simpson et al., Neurochem. Res. 1996, 21(10):1221–1226; W. Huang and R. K. Simpson, Neurological Res. 2000, 22:160–164).